The Equatorial Sextant , also known as Altitude Instrument, was first invented and made by William Austin Burt. He patented it November 4, 1856, in the United States as patent No. 16,002, which was followed by patents in England, France and Belgium. The purpose of this type of sextant was to get an accurate position of a ship at sea. Burt applied the principles of his earlier Solar Compass invention to navigation.
Equatorial sextant Wikipedia
One day in 1851 on a return trip to Philadelphia from Europe Burt intentionally took back to America a slower sailing vessel. He wanted to observe what happened out at sea and took notes on this longer ocean trip. He wanted to apply ideas he had to his Solar Compass invention to make a guide for sailors at sea, like he had done for surveyors in the wilderness. Burt noticed on this six week trip that the captain was misguided by the ship's compass, which caused the ship to go off course several times delaying the arrival. He applied his findings and experiments of this return trip to his Solar Compass with the idea to help navigation. The resulting instrument lived up to his expectations. He had considered labeling the new instrument with the names of "Marine Solar Compass", "Computing Sextant", and "Pantalobe" but settled on "Equatorial Sextant" as the original compass was a derivative of the reflecting sextant. He gave to sailors that had no path a means of navigation, just like he gave to surveyors a path to follow when they had no navigation when their normal instruments failed due to large iron ore deposits.
Shortly after he had designed his Equatorial Sextant he retired as a surveyor to devote his time to teaching the use of this special navigational sextant. He then moved to Detroit for this purpose. Burt & Bailey company of Detroit built Burt's first prototype by the early part of 1855. Burt had obtained an accurate reading within five degrees on board the ship Illinois on its historic first journey through the Soo Canal on June 18, 1855.
In March 1856 Burt sent a sample of one of his Equatorial Sextants to the Navy Department. He sent another sample to the U.S. Coast Guard. They both agreed that Burt's Equatorial Sextant met the purpose of its design. His precision instrument was then finalized and approved on a patent November 15, 1856. Burt envisioned a greater worldwide potential for his nautical instrument than for that of his surveying instrument of the Solar Compass.
Burt instructed lake captains in the use of this new sextant and gave them classes in astronomy and navigation. The captains learned valuable knowledge taught from "old salts" and they were able to make successful winter trips across the Atlantic with just their lake schooners. Burt died, however, two years after he patented the Equatorial Sextant and he was not able to perfect his new navigational aid.
Burt's instrument was used to take azimuths, altitude, time and declination with one observation when properly manipulated. There were two principal rings. The inner latitude ring could rotate 90 degrees inside the outer meridian ring. An azimuth ring was attached to the meridian ring. A ship's bearing was determined by settings of the astronomical triangle consisting of declination, latitude, altitude, hour angle, and azimuth. Three of these were used. The main purpose of Burt's new instrument was to determine the variation of the needle in the magnetic compass for a more accurate reading of a ship's position at sea.
Burt claimed in his patent was he took the common sextant and combined it with his special mechanical techniques of horizontal and equatorial movement settings to obtain latitude, time, azimuth, altitude and declination without having to figure it out mathematically as they were read directly off the instrument. Many problems in nautical astronomy are solved at once with the use of the Equatorial Sextant. It has the capability of reading off the latitude, hour angle, and azimuth without computation thereby eliminating mathematical errors. It can do these readings any time of the day. It is a combination of the reflecting sextant with meridian, azimuth, and hour circles. It is designed to give immediately the azimuth and hour angle by observation of the altitudes of the heavenly bodies. The corresponding angles are read off on the respective circles, thereby giving the position of a ship at sea at once with the use of a marine chronometer.